Transmission system



Feb. 13, 1945. R, H. HERRICK 2,369,

TRANSMISSION SYSTEM Filed April 19, 1943 s Sheets-Sheet 1 FIG. I

llO

INVENTOR. ROSWELL H. HERE/GK ATTORNEYS Feb. 13, 1945 H, HER..K 2,369,460

TRANSMISS ION SYSTEM Filed April 19, 1943 3 Sheets-Sheet 2 INVENTOR. ROSWELL H. HERRICK ATTORNEYS Feb. 13, 1945. R. H. HERRICK TRANSMISSION SYSTEM Filed April 19, 1943 5 Sheets-Sheet 3 LINE INVENTO'R ROSWELL H. HERRIGK rromvsvs Patented Feb. 13, 1945 TRANSMISSION SYSTEM RoswellH. Herrick, Oak Park, 111., 1assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Application April 19, 1943, Serial No. 483,596

nclaims. ('01. 179-81) The present invention relates generally to improvements in signal current transmission systems of the type in which signal controlled switching circuits are provided for partially or fully blocking, under certain conditions, certain of the signal channels included therein and to increase the gain of certain of theother signal current channels, and, more particularly, to improvements in telephone substation circuits having coupled signal currentchannels for the transmission of incoming and outgoing signal cur rents. p 1 l Heretofore, telephone substation circuits have been provided with a hybrid system or anti-sidetone impedance networks for preventing signal currents developed during operation ofthe transmitter from being transmittedto the receiver and for similarly preventing signal currents incoming over the line extendingto the substation from being transmitted to the transmitter for reproduction. In such arrangementone of the factors which determines the efficiencyof the sidetone suppression is the impedance of the talking circuit established by way of two connected subscriber lines. Since the impedance is not the same for any two difierent established connections, the hybrid system of each substation is usually balanced to provide maximum sidetone suppression efficiency for average line conditions. In installations wherein amplification of incomingand outgoing signal currents is required, as, for example, in executive loudspeaknot-in use, in other arrangements-the signal channel not-being used is renderedcomparatively in+ active. Apparatus of thetype in which the channels are only partially blocked whennotin use have the advantage ofpermitting conversation,

break-ins. V l

i In telephone. substation apparatus of the characterhaving amplifying channels, it is necessary to provide control circuits which are arranged to be exceedingly fast in operation so that speech:

This is partic- While in some arrangements the signal channels are completely blocked when clipping is minimized. In order to minimize speech clipping it may be found, however, that it will be necessary to reduce the time delay factor encountered bythe circuit equipment, but this makes the equipment more susceptible to feedback between the controlcircuits and be-. tween the channel amplifiers. It therefore would be desirable to provide a'relatively rapid acting control circuit arrangement which would minimize speech clipping andwhich, also, would retainthe advantages oftpreventing feedback be-q tween the different components of the apparatus. Heretofore the usual telephone substation circuits having channel amplifiers have been arranged so that the operation always included the use of such amplifiersQThis has a disadvantage in that it does not always provide the desired degree of Privacy for telephone conversation and, hence, it would be desirable to provide an arrangement whereby either a loudspeaking type of communication or the usual telephone type ofcommunication could be obtained selectively. Furthermore, it would be desirable if an arrangement were provided whereby rapid change-over could be made between the two types of conversation, without appreciable interruption.

It is an object of the present invention to provide an improved telephone substation apparatus which is adapted to be operated either as a conventional telephone or as a loudspeaking telephone. l

It is another object of this invention to, provide an improved transmission system particularly suited foruse in telephone substation circuits, wherein a plurality of channel amplifiers are utilized and wherein the control circuits are arranged to be exceedingly fast in operation to minimize speech clipping and yet maintain adequate provisions for the preventi-on of feedback. Still another object of the invention is to provide a pair of channel amplifiers for use inthe loudspeaking set which may be conditioned for use in a minimum of time, whereby it will not be 1 necessary to continuously, energize the amplifiers.

A still further object of this invention is to provide an improved transmission system for use in' a telephone substation circuit, whereby during a conversation a'rapid changeover may be made between the conventional telephone set op-' eration and a loudspeaking setoperation.

A still further object of the present invention is to provide, an improved transmission system whereby executiv loudspeaking telephone operation may be provided to ether with regular subset operation which willpermit the use of conventional subset equipment and which, furthermore, when used with an automatic telephone exchange will permit dialing for either type of service.

In the illustrated embodiment of the invention there is provided a, telephone substation circuit of the type adapted to be connected to an associated line, which comprises the usual transmitting andreceiving means. The circuit also includes a transmission system comprising a hybrid system or network, a first channel for transmitting signal currents from a microphone through the hybrid system to the associated line,'and a 1 second channel for transmitting incomingsignal currents from the hybrid system to a loudspeaker.

receiving means. In accordance with the present invention there is provided a switch for condie tioning the channel amplifiers of the loudspeaking equipment for operation. Each of the chan-:

nels is provided with a control circuit responsive to signal currents passing through the channel for increasing the gainof that channel and for decreasing the gain of, the other channel. Each of the channels is also provided withan automatic volume control having a long time constant in order to maintain the gain of the amplifier so as to hold over the word andsyllable pauses. In order. to provid an exceedingly rapid enabling and disabling action by the controlcircuits interconnecting the amplifiers, any arrangement is provided whereby the initial control voltage apof direct current. Such an arrangement avoids delay, eliminates theneed for continuous operation, and prevents feedback between the various control circuits and channel amplifiers. v o vEach of the control circuits in response to signal currents through the associated; channel, generates control voltages which initially are applied to the associated channel with a. minimum of filter action. A slow releaserelay operates tov in-- creasethe negative bias on the other channel and also. operatesto switch additionalfilter means which comprises a capacitor, initially charged to a certain voltage, to the means for deriving control voltages fromthe control tube. By providing means forinitially charging the. capacitor, the additional filter action, which of course introduces a time delay, is connected to th control circuit without appreciably affecting the derived control voltage. 1 Other and further features of this invention will subsequently become apparent in the followingdescription. v

'The novel features believed to-be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objectsand advantages thereof, will best be understood by reference to the following description taken in connection withthe accompanying drawings, in which Figs. L te- 3, inclusive, illustrate the substation circuit "set. communication, which will subsequently become having incorporated therein certain of the feature's'of the invention as briefly outlined above.

Referring more particularly to Fig. 1 of the drawings, there is shown a telephone subset comprising a transmitter I0 I, a receiver I02, a receiver switch I03, induction coils I08, I09 and III, and

an isolating or coupling capacitor H2, The subset may be provided with a dial switch H3 and also witha ringer I22 and a ringing capacitor I 24. These parts constitute the conventional ele- -merits commonly found in a subscriber telephone subset. The standard subset is arranged for selective' use with an executive type loudspeaking The selective operation of either type of apparent, is brought about in part by a switch I 25 whichioperates to, condition for operation the louds'peaking equipment. The loudspeaking equipment includes a microphone I33 and a loudspeaking telephone I35. v The telephone transmitter IOI, the receiver, I02, the induction coils I08, I09 and III, the calling device H3, and the receiver switch I03 are interconnected to form a substation circuit'of the well known anti-sidetone type. This circuit includes the line conductors H0 and I20 betweenwhich the three inductively coupled windings I08, I09 and I I I are serially included in a branch circuit. This branch circuit also includesan isolating or couplin capacitor I I2 connected between the two windings I08 and III, the capacitor being effective to prevent the receiver from being energized by the direct current flowing .over the conductors H0 and I20 when the substation is connected for use. The portion of the branch circuitincluding,

nected between the two windings I09 and III, it.

is protected against high voltages appearing upon the-line comprising the conductors IIOand I20, which voltage surges might-result in puncturing the insulation of the capacitor. r

For the purpose of giving an audible indication of calls incoming to-the substation, there is provided'a signal device in the form of a ringer I22 which is normally connected to be energized over an operating circuit including the line conductor I20, the capacitor'l'24, the contacts H4 and H5 of the calling device II3,'and the contacts I3I and I32 of the switch I25 whicha're'connected to the other line conductor II0.- The induction 0011 I08 which has one terminal connectedto the juncture between the transmitter I01 and the the resistor I 23- will' subsequently become appar-" -ent.-

The calling device H3 is provided witha make contact II6 which cooperates with the contact II5 which, it will be remembered, was connected to the capacitor I24. The ca-lling'd'evice, furi thermore, hasa pair of make contacts II! and H8 which are connected between the lineconductor III] and the common-juncture between-the resistor 123, the inductance coil I08 and the switch contact I28 of the switch 125; Upon actu ationof the-calling device I13 a pairoi'interasea'ceo rupter' contacts H9 and I2I are arrangedto be broken to provide the calling impulses. These contacts are connected between the switch contact I I8, which is connected to the commonjuncture of the resistor I23, the inductance coil I08, the switch contact I28, and the other line conductor I20. When the dial or calling device I I3 is actuated the ringer I22 is disconnected from the conductors H and I20 which comprise the telephone line, and the capacitor I24 is connected through the resistor I23 so as to be inparallel to the interrupter contacts H9 and I2I, thereby to prevent or minimize transient efiects occurring as a result of the making and breaking of the contacts H9 and I2I. The calling device H3 is. arranged to be effectivewhenever the hook or receiver switch I 83 has been operated to the posi.. tion shown whereby the contacts I04 and I05 thereof are closed so as to complete the circuit from the contact I I9 of the calling device through the inductance coil I 08 and the transmitter IOI to the contact I04. At the same time that the con. tacts I04 and I05 are closed, contacts I06 and I01 have also been closed so as to prepare a circuit which is arranged to operate to render ineffective the hybrid system connected to the telephone line H0 and I if the loudspeaker set is in operation. This circuit operation alsowill subsequently become apparent in the following description. I The contacts I06 and I01 of the hook switch I03 are connected to the make contacts I26 and I21 of the switch I25, the latter contact being connected to the conductor I40 which, in turn, is connected to control a relay to perform the operation mentioned. i

The calling device ordial mechanism I I3 is also arrangedto become efiective upon actuation of the switch I25 which will close the make contacts I28 and I29, the latter contact being con--v nected to the conductor I30 which is connected to a hybrid system interposed between theline conductors IIO and I20, and the two channels connected to the microphone I33 and the loudspeaker I35. Upon actuation of the switch I25 the break contacts I 3I and I32 are opened so as to disconnect the ringer I22 from across the line conductors H0 and I 20.

By referring to Fig. 2 as well as Fig. 1, it will be seen that the microphone I33 is connected to a pair of conductors I 50 and I 60 which are connected to the primarywinding of the transformer 20I. A pair of resistors I34 and I35 are conpling resistor 2I1 and a coupling capacitor 222.- The coupling capacitor 222 is connected to the control grid of an auxiliary vacuum tube or control tube 224. I I I The other terminal of thesecondary winding of the transformer 20I is connected through resis-' tors 285, 281,283 and 28I toground. The juncture between the resistor 285 and the secondary winding ofthe transformer MI is provided with a capacitor 286 which, in turn, is connected to ground. The juncture between the resistors 285 and 281 is connected to a capacitor 288 which also terminalof the anode coupling resistor 203, and

pacitor 2I9. tube 2I6 is connected to a variable resistor, 225

nected'across the conductors I and I and the common juncture thereof is connectedthrough a signal lamp or device I36 to ground- The primary winding of the transformer 20I is provided with a. mid-tap which is connected to a conductor 2I0.' The conductor 2I0 is connected to a contact 302 which cooperates with a make contact 303 of a relay 30I, seen in Fig. 3. The outputwinding of the transformer 20I has one terminal connected to. the control electrode for grid of a vacuum tube 202. The anode of the vacuum tube 202 is.connected to one terminal of a coupling resistor 203 and to a coupling capacitor 206 which, in turn, is connected to a grounded coupling resistor 201 which has a movable con-.- tact connected to the control grid of a vacuum tube 208. The anode of the vacuum tube 208 is connected to one terminal of a coupling resistor 209 and to a coupling capacitor 2I4. The coupling capacitor 2| 4 is connected to one terminal of a grounded coupling resistor 2I5 and to the controlgrid of a vacuum tube 2I6. The anode of the vacuum tube 2I6 is provided with a con jby-pass capacitor2l I is provided between ground and the screen grid of the vacuum tube 208.. .The

vacuum tube'208 is provided with a self-biasing arrangement comprising a resistor 2I2 between the cathode and ground which, in turn, is bypassedwby thewcapacitor 2I3. I l The anode of the vacuum tube 2 I6 is connected indirectly to the high potential conductor 280.

The vacuum tube2I6 is provided with a cathode resistor 22I connected between the cathode and ground, and the resistor is by-passed by a ca- The screen grid of the vacuum which is connected to the cathode of the vacuum tube 224. A grid coupling resistor 223 is con nected between the grid of the vacuum tube 224 and the resistor 225. The outgoing channel amplifier comprising the vacuum tubes 202, 208 and 2I6 togetherwith,

tive to build upa voltage across the resistors 283 and 284. A portion of the voltage appearingacross these'resistors is impressed upon the grid of the vacuum tube 202 through the resistors 281 and 285 and the secondary winding of the transformer 20I.

The capacitors 2815 and 288 which are connected between ground and the juncture between the resistor 285 and the transformer HI, and the juncture between the resistors 285 and 281, respectively, together with the resistors 285 and 281, form a filterwhich prevents voice feedback through the automatic gain control system. These components are so selected as to give a comparatively long time conanode of the vacuum tube .224 is provided with a coupling resistor 226 which, in turn; is con nected to the common juncture of a cy-Pass capacitor .292, the contact 236 of a relay 233,3 resistor .228, and the filament of the vacuum tube 224.. The resistor 2.28 is serially connected a..re1ay 229 between the filament of the vacuum tube 224'andground. The capacitor 292 which is connected from one terminal of the resistor 5225 to ground serves as a icy-pass capacitor for this resistor. The relay 229 is provided with make contacts 231 and 232, the latter contact connected to the conductor 21:0. The contact 23! or the relay 229 is connected to a slow release relay 233 which, in turn, is connected to ground. The slow release relay 233 is provided with a pair of break contacts 234 and 235 which are arranged to remove a short-circuit across a resister in the cathode .circuit of the first tube 242 of the incoming channel of the other amplifier, therebyto increase the negative bias and thus to disable the amplifier. The slow release relay 233 is, furthermore, provided with a pair of break contacts 238 and 23H, the latter contact cooper:- ating with the make contact 235 so as to switch a capacitor 239 connected between ground and the contact 1231, from the contact 238 to the :contact 23.6.. The break contact 233 is connected through a resistor .244 of the high potential conductor 280 so as to charge the capacitor 239 to a predetermined potential before it is connected to the contact .236. The contact 236, it will be remembered, is connected to the filament of the cathode 224 and to the biasing resistor 225..

The voice operated gain control operation per:

formed by the vacuum tube 224 :is such mat the outgoing channel is rapidly provided "with an increased screen grid potential applied .to the output vacuum tube 2H5. One of the serious objections heretofore enco ntered in direct voice operated gain control of amplifierslby a vacuum tube is the tact that the anode or plate current or the control tube inherently has voice cornponents which .must be thoroughly filtered out in onder to prevent amplifier feedback between the amplifiers through the circuits associated fol-lowing a pause in speech. In the present arrangement this difiloulty 201 disadvantage is obviated by initially applying to the screen grid of -the output .tube 216 of the outgoing channel a control volt-age obtained from the biasing resistor-225 which has but a minimum amour-it m filter comprising the capacitor .282;- After initial voltage appearing across the resistor 2.25 has been applied to the screen grid of the out .put tube 2|"6 of the outgoing channel, it is then trol tube are described.

possible to introduce additional filter'iactifinfbi connectmga capacitor, such as the capacitor 239, to one terminal of, the biasing resistor 225.. If a capacitor which is uncharged is then applied so as to introduce the extra filter action, there may be .a momentary change in the gain of the amplifier which would be undesirable. To obviate this possibility the capacitor 239 is maintained in a charged condition by being connected through the resistor 24! to a suitable source of potential which is supplied from the conductor 280. B7

switching the. charged capacitor 239 to the re sister 225 by means of the slow release relay 233., there will be no appreciable change in the gain of the outgoing amplifier. it

While the control tube 224 andthe circuit olements associated therewith operate to applyan initial and relatively unfiltered control voltage to the screen gridof the outgoing amplifier tube 215, and subsequently a filtered voltage, thereby to increase the-gain of the outgoing channel, the control circuit also operates to decrease the gain of the incoming channel, but this operation will be better understood if the description thereof is deferred until the incoming amplifier and its con The incoming amplifier channel is connected to receive signal currents from the hybrid system transformer 304 through the conductors 3-2-0 and 213 0 which are connected across the resistor 243.

An adjustable contact on the resistor 243 is connected to the control grid of the vacuum tube 242. The juncture of the resistor 243 and the conductor 3201s connected to one terminal of 1a mon juncture between two resistors 246 and 247 which are connected between ground and the cathode of the vacuum tube 242. .These resistors are in parallel with a capacitor 248 and another biasing resistor 249. The screen grid of the vac.-' uum tube 242 is connected through a series re-' sis'tor 252 to the high potential conductor 280'; The screen grid of the vacuum tube 242 is bypassed to ground by the capacitor25l. Anode potential is provided from the conductor280 through the anode coupling resistor 2'53. The

anode of the vacuum tube "242 is coupled through a capacitor 254 and a resistor255 t0 the grid of the vacuum tube 259. The grid of the vacuum tube 259 is connected through .a resistor 256 to ground. .The cathode of the vacuum tube 2.59 is provided with a self-biasing resistor 258 which is provided with aby-pass capacitor 251, these elements :bejin'g -.connected between the cathode and ground. The anode ;.of the vacuum tube-2.5.9, is connected through the primary winding of .the transformer 28.2 to the conductor 28.0 which profv viides suitable anode potential. The output wind ins. of the transformer 2.02 is connected to the conductors .lfluand 189 which, in turn, are com nectefllto energize theloudspeaker 13.5 or rug. 1. '.".'Ihe. in.comin channel amplifier which comprises the vacuum tubes 242 and .259 together with the associated circuit elements is provided with .an automatic gain control obtained from the now .of grid current of the input tube 254.2.- Grid current flows whenever the incomin ice or signal volta e appearing etween the condu t tors .320 and 330 reaches a predetermined value; This grid current hows throughthe resistors 12 4:3. and '245, the latter resistance being much greater than the former resistance, with the capacitor 244 acting as a filter and storage reservoirto smooth the rectified half wave signal or voice voltage into an average direct current voltage proportional to the signalor voice ,wave amplitude. Thus it will become apparent that the bias applied to the vacuum tube 242 is a function of the signal or speech amplitude and the gain of the vacuum tube 242 is an inverse function of the grid bias.

Further control of the gain of the. incoming amplifying channel is obtained from the action of a control tube 291 together with its associated circuit elements. The coupling capacitor 261 is connected between the anode of the vacuumtube 259 and the grid of the vacuum tube 291. A grid coupling resistor 263 isconnected to an adjustable resistor 264 which is connected between the filament current supply conductor 230 and the cathode of the vacuum tube 291. The screen grid and the anode of'the vacuum tube 29] are connected to the conductor 210 which is connected to a suitable source of anode potentiaL. A variable anode-coupling-resistor 265 is connected between the anode and the cathode of the vacuum tube 291. The conductor 210 is connected to a contact 211 which is arranged to complete a circuit with a contact 269 of a relay 261, thereby.

to energize a slow release relay 212 connected between ground and the contact 269. A suitable capacitor 266 is connected between the cathode and ground, for by-passing the bias resistor 264. A relay 261 is serially connected with a resistor 268 between the cathode of the vacuum tube and ground. The screen grid of the vacuum tube 259 is connected to the cathode terminal of resistor 264 andalso to the contact 215 of the slow release relay. 212. The contact 215 is arranged to cooperate with the contact 216 which is normally inmake position with the contact 211. The contact 216 is connected to one terminal of the capacitor 218, the other terminal of which is connected to ground. The contact 211 is connected through a resistor 219 to the high potential conductor 2811, thereby to provide a circuit a resistor 289. The other terminal of the resistor r 3289 is connected to'the common juncture of. the

resistors 283, 264 and 281-which are associated with the control grid circuit of the vacuum tube 202.

" When voice or signal currents are received by u the incoming channel, a portion of the output voltage of the vacuum tube'259 is supplied by the capacitor 261 to the control grid ofthe vacuum tube 291. The increased anode to cathode current flowing as a result of the application of such grid voltage produces avoltage drop across the resistor 264 which is applied to the screen grid of the vacuumtube 259, much in the same manner as the voltage appearing across the resistor 1225 of the vacuum tube 224- was applied to the screen grid of theoutput-vacuum tube 216 of the outgoing channel, thereby to increase the gain of that channel. When increased anode to caththe vacuum tube 259, thereby to provide sufficient the channels. At the same time, the relay 212 closes the contacts 213 and 214, thereby connecting the resistor 289 to ound. his as the effect of completing a voltage divider circuit comprising the resistors 263 and 269 which are effectively bridged across the resistor 281. This produces an increase in the bias applied to the control electrode of the vacuum tube 202. This in: crease in bias thereby disables the outgoing. channel amplifier by reducing the gain thereof.

The incoming channel is disabled by operation of the control tube. 224' by action of the slow release relay 233 which operates to open the break contacts 234 and 235. The opening of these contacts removes the short circuit appearing across the resistor 246, thereby increasing the negative bias on the input vacuum tube 242 of the incoming channel. The use of slow release relays 233 and 212 serves tomaintain the enabling and disabling functions over the word and syllable pauses of speech. Reference may now be had to Fig. 3 in conjunction with Fig. 2, wherefrom it will be seen that the incoming telephone line comprising the conductors and 120, in addition to being connected to the telephone apparatus of Fig. 1, has one of its conductors 110 connected to a hybrid system which may be of any desired type, although it may be preferable to utilize the system' shown which includes two transformers 304 and 305, each having two secondary'windings 366, 311 and 301,309 respectively. One second ary winding 306 and 301 of each transformer 304 and 305 isarranged to be connected across the telephone conductors 110 and "120. Such circuit connection may be traced from the conductor 1 10 through thesecondary winding 306, a relay coil 313 shunted by a capacitor 308, the secondary winding 301, break contacts 319 and 321 of relay 316, the conductor to the contact 129 of switch 125 of Fig. 1. Actuation of switch 125 will close contact 129 with contact 128 to complete a circuit through contacts 119 and 121 of the calling device 1 13 to the other line conductor 120 where-'- by the hybrid system will be connected to the telephone line. The transformer 305 has a secondarywinding 309 which is connected in oppo sitio-n to the secondary winding 311 of the trans-i former 304 through a resistor 312. The two transformer type of hybrid system. has the ad vantage in that the desired high impedance windings may be rapidly obtained as compared to a single transformer hybrid coil. Thisresult's in a better balance so as to' prevent loudspeaker response during'outgoing speech. j j

The primary winding of transformer 305 is connected between the conductor 290 and the re lay 301 which, in turn,'is connected to the conductor 280. It will be remembered that the conductor 290 is connected to the anode of the output tube 216 of the outgoing channel and that.

the conductor 280 is connected to a suitable source of potential whereby as soon as the amplifier is conditioned for use the rel'ay 301 willbe actuated to close its contacts 302 and 303. The-conductor 240 is connected to the contact 303 so as tosupply energy overthe conductor 210 to the midpoint of the transformer 201 'of Fig, 2, thereby to complete a simplex circuit across the microphone conductors and to illuminate the signal lamp 136, thereby indicating that th'e amplifiers are in'co'ndition for use. The conductor 130 whichisfconnected through the confiltering action as to prevent feedback between it;

tacts 319 and 321 to one terminal of a hybrid Winding 301' is'connected to the contact 129 of the switch I25 of Fig. 1, so-that when' this switch is actuated to close the contacts I26 and I29 a circuit will be completed to the line conductor I26, thus causing current to flow through the windings 366 and 361. The relay 3I3 connected between these windings is actuated by the current flowing and this relay closes contacts 3I4 and 3| 5. Contact 3I5 is connected to one side of an alternating current source 324 and contact 3 I4 is connected through a relay 362 to the other side of the alternating current source 324. The relay 362 thereby is actuated to close its contacts 363 and 364 which are connected in series with the primary windings 325 to 321 of a plurality of power transformers.

The transformer winding 325 is provided with 'secondary windings 328 and 326 which are connected to provide cathode and anode voltage for a full wave rectifier valve 33L The midpoint of the secondary'winding 326 is connected to ground and the power terminals of this winding are con-' nected to the anodes of the valve 33I. The cathodes of the valve 33I are connected through a choke coil 332 to the conductor 216. Suitable filter capacitors 336 and 333 are connected between ground and opposite ends of the choke coil 332. The rectifier valve 33! also supplies another voltage obtained from a bleeder circuit comprising resistors 334 and 335 connected between the oathode and ground. The desired potential is obtained at the juncture of these resistors and the power is fed through two choke coilsor filter reactors 331 and 339 to conductor 286 which supplies potential to the anodes of incoming channel tubes 242 and 259. A filter capacitor 338 is connected between ground and the two choke reactors. A second filter capacitor MI is connected between. ground and the juncture of choke coil 339 with conductor 286.

' A transformer having the primary winding 326 is provided with two secondary windings 342 and 343, each of which is connected to full wave rectifiers 344 and 348, respectively. Each of these full wave rectifiers may be any of several types well known in the art, although it has been found preferable to utilize rectifiers of the dry-contact type. The output terminals of the rectifier 344 are connected to the conductors 250 and 266 through a choke coil 346. Suitable filter capacitors 345 and 341 are connected across the conductors 256 and 266 at opposite ends of the filter reactor 346. The conductors 256 and 266 of the full wave rectifier 344 serve to energize the filament of-the control tube224 which is associated with the outgoing channel amplifier. One output terminal of the rectifier 348 is connected to ground and the other terminal is connected through a choke coil 35I to the conductor 246.

a capacitor 346to ground and the other terminal of thechoke coil 35I is connected through a capacitor 352 to ground. The conductor 246 is conground and the other terminal. is connected through the choke coil-356 to the conductor 3I6 which, in turn, supplies gfilament'current to the vacuum tubes 242 and 259 the incoming amif r chan els. Qne t rmin l-9 e choke o 356 is connected to acapacitor-35'I and the other 26 l r H For normal handset telephone operation,:the

One ter- 'minal of the choke coil 35I is connected through ac'ccnco terminal of'the' choke coil is connected to ace), pacitor 358, both capacitors, in turn, being con-'- nected to ground. The output terminals of the rectifier 359 are connected to the conductors 226 and 236 through the choke coil 36I. Capacitors 362 and'363 are connected across the conductors 236 and 226 on opposite sides of the choke coil 36I. The conductors 226 and 236 supply filament current to the control tube 25 which is associated with the incoming amplifier channel. It will be.- come apparent that each group of tubes for the various signal channels and each control tube is supplied from a separate source of filtered direct current, whereby a low thermal capacity typeof vacuum tube may be employed so that the amplifiers need not be energized continuously. The isolation of the various groups of tubes and the tubes from each other has the advantage of Dre-.- venting feedback through the filament circuits of these tubes.

receiver I62 is lifted so as to permit the receiver or hook switch I63 to be actuated to the position shown in Fig. 1. This connects the transmitter and receiver into the circuit so that a dialing tone is heard. Thereupon the calling device or dial H3 is actuated so as to call the desired subscriber party. The subscriber handset is arranged to be active any time it is lifted from its cradle whether the loudspeaker equipment was previously active or not. A line circuit is completedby actuation of the receiver switch I 63 from the conductor II6 through the contacts I64 and I65, the transmitter opening its contacts 3I9 and 32I, and closing its contacts 322 and 323. The opening of the contacts 3I9 and 32I operates the disconnect of the hybrid coil system which interconnects the line conductors H6 and I26 with the two loudspeaker channel amplifiers. The contacts 322 and 323, when closed, short circuit the input to the vacuum tube 242 of the incoming channel amplifier. The switch I25 having been actuated to condition the channel amplifiers for operation also has opened the contacts I3I and I32, thereby disconnecting the ringer I22 from its normal position in series with the capacitor I24 across the lines no and I20.

If it is assumed that the telephone handsetis resting on its cradle to actuate the switch I63 to the open-circuit position, and it is desired to communicate by means of the loudspeaking executive telephone apparatus, the switch I25 is actuated, thereby connecting the calling device II 3 across theline by the closing of the contacts I28 and I29. The contact I26 is connected to the conductor I36 which, in turn, is connected through the contacts M9 and 32I, which nor- 325, 326 and 321. Poweris therefore supplied to the vacuum tubes 202, 208, 2I6',224, 242, 259 and 29I. Theuanode of the output tube 2I6 of the outgoing channel amplifier is connected to the conductor 290 which, in turn, is connected through the hybrid system winding 305 and the relay 30I which, in turn, is connected to the conduotor 280. As soon as the vacuum tube 2I6 therefore reaches its proper operatingcondition, plate current begins to flow thereby causing the relay 30I tobecome energized so as to close its contacts 302 and 303, the latter contact being connected to the power conductor 240. Current supplied by the conductor 240 thereby ,fiows through the conductor 2l0 to the midpoint of the inputtransformer I, and over the microphone conductors I50 and I60 through the resistors I34 i from a charging circuit which includes the conductor 280, the resistor 2, and the contact 238 to the junction of the capacitor 292 with the one terminal of the resistor 225. Thus the capacitor ditional filter action there will be no appreciable and I35 to energize the signal deviceor lamp I36.

Thus it will be noted that a simplex circuit has been provided for the conductors I50 and I60; whereby an indication will be produced to show that the vacuum tubes of the communication channels for the executive loudspeaking apparatus have been conditioned for use. c a Conversation may then be started over the microphone loudspeaker system, without the use of any I manually operated talk-listen switch. Whenever the subscriber speaks into the micro? phone I33 signal currents are transmitted by the outgoing channel amplifier which includes the vacuum tubes 202, 208 and 2I6. By means of the coupling capacitor 222 connected to the anode of the output tube 2I6, the control vacuum tube 224 operates to produce control voltages appearing across the resistor 225 which supplies potential to the screen grid of the vacuum tube 2I6. When no signal currents are passing through the vacuum tube 2i6 the potential supplied by the resistor 225 is arranged to be sufficient for the reduced level of operation of the amplifier, and when signal currents are being transmitted voltage appearing across the resistor that the plate current of the control tube, suchas 224, will inherently have voice components which must be thoroughly filtered, out in order to prevent feedback between the amplifiers of the two channels through the circuits associated with the control tubes, such as the tubes 224 and 29I.

When the amplifier between the control and controlled points has a considerable gain, the degree of filtering must be'one which inherently results in a long controlling time constant which produces an appreciable delay between the application of the signal voltage and the enabling action which increases the gain of the channel amplifier. The filtering, needed for even moderately high gain amplifiers produces sufiicient delayto introduce a serious degree of clipping or suppression of the first portion of each word following a pause in speech. In order that theincreased voltage applied to the vacum tube 216 may be applied withconsiderable rapidity and without great delay, a minimum of filter action is provided by the by-pass capacitor 292. After the initial coupling action has been applied to the output tube 216 the control tube 224 through the actuation of the relay 229 which, in turn,

brings about the operation of the slow release re- 1ay233, causesa capacitor 239 to be switched change in the gainof the amplifier. I 7

At the same time that the additional filtering action is provided for the resistor 225, the con tacts 234 and 235 of the slow releaserelay 233 open a short circuit across the resistor 246 which appears across the biasing resistor 249 of the cathode to ground circuit of the vacuum tube 242, which is the first stage of amplification for the incoming channel amplifier. short circuit from the resistor 246 increases the negative bias on the vacuum tube 242 so as to appreciably reduce the gain of the incoming signal ,channel amplifier. The outgoing signal channel amplifier is also provided with an automatic gain control voltage which is derived through the coupling capacitor 2I8 and the diode portion of the vacuum tube 208, and this voltage appears across the resistors 283 and 284. A portion of this voltage is applied through the resistors 285 and 281 to the grid of the vacuum tube 202. The resistors 285 and 281 together with the associated by-pass capacitors 286 and 288 provide a filter arrangement having a sufiiciently long time constant as to hold over the gain control voltage over the word and syllable pauses of the outgoing speech, thereby giving a smoother andmore positive control. I I

If incoming signaling currents are received across the conductors 320 and 330, they impress a voltage across the resistor 243 which isamplified by the Vacuum tube 242, which, in turn, is coupled to the output vacuum tube 259. A portion of the output voltage appearing at the anode of the vacuum tube 259 is; coupled by the capacitor 26I to the grid of a control tube 29L A resistor 264 is connected to the screen grid of the vacuum tube 259 and the cathode of the control tube 29! so that, with no signal currents appearing at the anode of the vacuum tube 259,

the proper amount of voltage is applied to the screen grid of that tube. When, however, signal currents are being received by this channel, the impression of a. portion of the signal currents on the grid of a control tube 29I causes additional anode current to flow, thereby rapidly increasing the potential acros the resistor 264, 5 thereby quickly increasing the sereengrild potential on the vacuum tube 259 to increase the gain thereof. The increase in flow out the current of the control tube 29I brings about energization of the relay 261 so that the contacts269 and 2H thereof are closed to energize the'slow release relay 212. The slow release relay 212 thereupon switches in additionalfiltering means compris- The removal of a time that the additional filtering-action is pro; vided to prevent feedback through'thecontrcl tube 291,: the slow release relay 212 closes its contacts 213 and 214, thereby connecting a re-; sistor289. toground; This has the effect of completing what amounts to a voltage divider circuit comprising the resistors 283, and 289 connected in parallel to the resistor 28l, thereby delivering increased negative bias to the vacuum tube 202, thus'disabling the outgoingsignal channels.

.The incoming signal channel is also provided with an automatic gain control action which is obtained in the following manner. biasing voltage for the vacuum tube 242 is obtained due to the ilow of grid current whenever incoming signal currents are received.- The vacuum tube is arranged so that: the grid. cur rentfiows-whenever theincoming signaling currentexceeds' a predetermined peak value which is greater than a certain bias for the tube. This grid current fiowsthrcrugh the resistors 243-,an'd 245, thelatter resistor having a rather high value, withthe capacitor 244 acting as a filter and storagereservoir serving to smooth the rectified half waves Olf' the signal into an average direct cur-' rent voltage of a value proportional to the signal Wave amplitude. Inasmuch as the built up bias is the function of the speech amplitude, the gain of the vacuum tube 242 is an inverse function of the impressed grid bias and, hence, its gain becomes an inverse function of a speech or signal amplitude, with the result that an automatic gain control action is obtained sufficiently close in degree to be satisfactory for the purpose.

From this description of the operation it will become apparent that each channel amplifier is provided with a control tube responsive to the signal currents flowing therethrough, whereby the gain of a signal channel receiving signal currents is initially rapidly increased by an apparatus having a minimum of time delay and filtering action, and subsequently suflicient filter in action is provided to prevent feedback between the channels. At the same time the other channel amplifier is disabled by supplying thereto a control voltage, usually. in the form of a negative bias,"thereby to reduce the gm'n of the unused channel amplifier. The automatic gain control of each channel amplifier is arranged to have a sufliciently long time constant so as to hold over the word and syllable pauses.

In order that the amplifier may be conditioned for use in a minimum of time, tubes are employed or the filament type which have a relatively small thermal capacity so as to heat quickly; Low thermal capacity filaments, however, require extreme precautions against hum and feedback through the filament circuits.

Such precautions have been taken by providing by-pass capacitors such as the capacitors 282, H3, H9, 248 and 251, and by also isolating each group of tubes for each of the channels from each other and from each of the control tubes interconnecting the two channel amplifiers. The filaments of the tubes are supplied from power sources which supply direct current by means of contact re-ctifiers, the output of which is suitably filtered.

- "If, for example, it is desired to change from loudspeaker operation tothe handset, to obtain the advantage of privacy, which may be desirable if the subscriber has someone coming into his presence at the time, a mere lifting of the receiver to actuate thereceiver switch ")3 A negative brings about a disconnection of the hybrid sys: I

tem from the telephone line and a short circuiting' of the input of the; incoming signal channel; This action is so rapidthatall relay; operation takes place within a'timegof but a-few' milliseconds, the time consumed in moving the-hand:

set from the cradle to the subscribers ear.

:.While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein which are.

the loudspeaker and amplifier, said means in'-'- cluding. a relay controlled by said receiver switch for'disconnecting or, connecting said hybrid system when said receiver switch is closed or opened.

2. In a telephone subscriber station, the com bination comprising a telephone subset having a receiver switch, a loudspeaker, a microphone, an'amplifier'for said speaker, an amplifier for said microphone, a telephone line, a hybrid sys tem'for interconnecting said line with said amplifiers, manually operable switching. means for connecting said hybrid system to said line, means responsive to the connection of said hybrid system to said line for conditioning said-amplifiers for use, andmeans controlled by said subset receiver switch for disconnecting said hybrid system from said line whensaid amplifiers have been conditioned for use.

3. In a, telephone subscriber station, the combination comprising a telephone subset having a receiver switch, aloudspeaker, a microphone, an amplifier for said speaker, an amplifier for said microphone, a telephone line, a hybrid system for interconnecting said line with said amplifiers, a source of energy for said amplifiers, a

switch for connecting'said hybrid'system to said line and to cause said source of energy to be con nected to said amplifiers, and meansfcontrolled' by 'said subset receiver switch for disconnecting said hybrid system fromsaid line, said" latter means operating to disconnect said hybrid system only if said amplifiers are" energized.

4.In an automatic telephone subscriber as:

tion, the combination comprising a telephone subset having a receiver switch, an impulse call ing device, a loudspeaker and a microphone, an

amplifier for said speaker, an amplifier for said,

microphone, a telephone line, means connecting said impulse calling device to said line, a hybrid system for interconnecting said line wtih said amplifiers, manually operable switching means for connecting said hybrid system to said line, means responsive to the connection of said hybrid system to said line for conditioning said am plifiers for use, fmeansfor indicating the condi tion of' said amplifiers; and means "controlled by said subsetreceiver switch-.fordisconnecting said hybrid system from said line' when s'aid' ampli fiers have been conditioned'for use." I '5. Ina telephone subscriber statiornthe coma bination comprising a telephone subset having a receiver'switch, a loudspeaker, a microphone; an amplifier for said speaker, an amplifier for said microphone', a telephone line, a hybrid system for interconnecting said line with said ame plifiers, means .for' selectively, communicating,

by means'of said subset or-sald loudspeaker and microphone; comprising -a manually operable switch for connecting said hybrid systemto said line and for conditioningsaidamplifiers for use, and relay means for disconnecting said hybrid system from said line, said receiver switch be: ing arranged to connect said subset directly to said line and to energize said relay means when said amplifiers have been conditioned for use c 6. In a telephone subscriber station, the combination comprising a telephone subset having a receiver switch, an impulse calling device, a telephone line, said impulse calling device being" connected, to said line, a loudspeaker and a microphone, an amplifier for said speaker, an amplifierforsaid microphone, a pair of wires connecting said microphone to saidamplifier, a hybrid system for interconnecting said line with said amplifiers, switching means for connecting said hybrid system to said line and for conditioning said amplifier for use, means including a simplex circuit imposed on said;microphone conductors for indicating the condition of said amplifiers, and means controlled by said subset receiver switch for selectively disconnecting said hybrid system fromsaid line if said amplifiers have been conditioned for use. i

f 7. .A transmission system comprising a panic channels for transmitting signal currents in' different directions, each channel being provided with a vacuum tube amplifier, a control tubelfor eachamplifier arranged to be responsive to sigrial currents passing therethrough, means hav; ing a minimum of filter action associated with each of said tubes for deriving control voltages therefrom, means for applying said derived volt: ages to the corresponding amplifiers to increase the gain thereof, a relay for each tube connected to be energized thereby, means controlled bysaid relay for connecting additional filtermeans to said means for deriving control voltages, and means controlled by each tube for reducing the gain of the otheramplifier. I I 18, Atransmission system comprising a pair of "channels for transmitting signal currentsin dif:

ferent directions, each channelbeing provided with a vacuum tubejamplifier, an auxiliary vacu-Q tube for each amplifier arranged tofberesponsive, to signal currents passingth'erethrough,

means having a relatively short time. constant associated with each of said auxiliary tubes for derivingcontrol voltages therefrom, means for applying said derived control voltages to increase the gain of one of said amplifiers, a slow release relay for each control tube connected to be energized thereby, means controlled by said relay for connecting to said means for deriving control voltages, means for preventing feed-back between the amplifiers through the circuits of said auxiliary tubes, and means controlled by each relay for reducing the gain of the other amplifier.

9. A transmission system comprising a pair of channels for transmitting signal currentsin different directions, each channel being provided with a vacuum tube amplifier, a control tube for each amplifier arranged to be responsive to sigvoltages without; appreciably affecting the derived voltage, and means controlled by said relay for reducing the gain of the other ameplifier. 1 t i 10. .A transmissionsystem comprising a pairof channels for transmitting signal currents in different'directions, each channel being provided with a vacuum tube amplifier, an auxiliary vacuum,.=tube for each amplifier arranged to be responsive to signal currentspassing therethrough,

capacitor for each of said tubes, said relay being arranged-to be actuated by its associated, tube, means for charging said capacitor, said capacitor being arranged to be switched by said relay-from said charging means to said means for deriving control voltages, and means controlled by each relay for controlling the gain of the other amplifier.

l1. A transmission systemcomprising a pair of channels for transmitting signal currents in different directions, each channel being provided with a vacuum tube amplifier, a control tube for eaclramplifier arranged to be responsiveto signal currents passing therethrough, means having a minimum of filter action associated with each of'sa'id tubes for deriving control voltages therefrom, means for applying said derived voltages to the corresponding amplifier to increase the gain thereof, a'relay for each tube connected to,

be energized thereby, a capacitor'for each relay, means for charging said capacitor, said. capacitor being arranged to be switched by said relay from said charging means to said means for deriving control volta'ges,fand means nontrolled by each relay for increasing the negative bias of at least one of the tubes of the other channel amplifier.

12. A transmission system comprising a pair of channels for transmitting signal currents in different directions, each channel being provided with a vacuum tube amplifier including anout-" put tubel aving a screen grid, a control vacuum tube for each amplifier arranged to be responsive to signal currents passing through the output tube thereof, means for deriving from said controlitube'a control voltage applied to a screen grid of the output tube of its associatedamplifier, said control tube in response to currents passing through its associated amplifier being arranged toincrease the potential applied to the screen grid of the output tube, and means operative in response to signal currents passing through an amplifier for causing said control tube to apply to the other channel amplifier an increased negative bias, thereby to reduce the gain of the other channel amplifier. 13. A transmission system comprising a pair of channels for transmitting signal currents in difierent directions, each channel being provided,

with a vacuum tube amplifier, each amplifier being provided with automatic gain control means having an appreciable time constant to hold over i for applying said derived voltages to the corresponding amplifiers to increase the gain thereof, a slow release relay for each control tube connected to be energized thereby, meansfor pre- "14. A transmission system comprising a pair of channels for transmitting ssign'al currents in d'ifiere'nt directions, each channel being provided with a vacuum tube amplifiencach amplifier ing provided with automatic gain-control means having a time constantsufiicient to over word and syllable pauses acontrol tube for cach amplifier arranged to be responsive to signal currents passing Jthereth'reugh, means having an. relatively short time constant associated with each of said -=con-tro'l tubes for deriving 'control voltages therefrom; means for applying said derived control "voltages to the "output tube of "the correspond-ing amplifier to increase the Jgain thereof, aslow release relay fereachcontrol tube connected to be energized thereby, means each control tube for "preventing feedback between said amplifiers *inoluding .a capacitor;

being arranged to lee-"switched said relay from said charging means to saidmeansfor-deriving control voltages, and means controlled "by" each relay for reducing-the'gain Oftheptherammidifferent directions'each channelbeingprov'ided I with a vacuum tube amplifier having ancutput tube provided with a screen grid, each amplifier f being provided 'with automatic gain control means'having "a sufilcienttime constant to hold over word'and syllableipausesi'acontrol tubeand circuit for each amplifier arrangedto be 'respon sive to signal currents jpassing 'therethrough, means connected in said control tube circuit'fhaving a relativelyshortltime constantan'da mum of filtering action for deriving control voltages from said control tube, means for applying said derived control voltagesto. the screen g'i"id of the output tube of the correspondingiampl'ifle'r, to increase thegain thereof, auslowireleaserelay for each control tube connectedttobe' 1 increasefthe'gain'ofitsassociated amplifierantl therlaby, rimaans *for each-@control babe-circuit "preventing t'feedback between said amplifiers through said-circuit including *acapacitor; means for "charging-said capacitor to a "predeterminedpotential, said capacitor being arranged to be switched by saidrelaydromsaid chargingcrneains to'said means "for deriving-control voltages, and 'means' controlled by "each "relay for -reduclngthe gain or-the other "amplifier by increasing 7th negative bias or the input "tube thereo'f. I 16. transmission "system comprising a "pair (of channels for transmitting signal currents in diiferent directi'ons;each'channel beingprovitled with a vacuum tube amplifier,-a control'tubeanfi F's'ive 'to signal currents passing therethrough to torreduce the'igainmf the other amplifienffilter' in;v means for charging said capacitonsaid'capacitor Y means arrange clto'be connected 'to said control tzibe circuits to prevent feedback *betweenfsaid arm2iifiersthroughsaid circuits," each pf sai dnon trol" tubes and thetubes 'of said amplifiers having }small "thermal capacity filaments, "individual sources of 'filter direct currentior eachrof"said control tubes and the'itubestof each of said am plifiers, said sources of e'urrent'be'ing isolatedfrom each other to eliminate 'hum and ffeedback bet'ween said amplifiers and said control itubes through'the'filament circuitsthereof. "17. 'A"transm'i ssion "system comprismg a pair I of 'cl'1'annels "for "transmitting signal currentsfin diflerent' directions, each 'channel'being gprovided witha'vacuumtubeiamplifier, a control -tube and circuit for' each amplifier arranged "to be""re-; sponsi-veto signal currents; passing therethrouglfil means "for deriving control voltages :from'eacli,

control "tu'loeto control the gain of'its associated amplifier,"filter means connected to saiame ns for deriving control voltages thcrebyto'prevent feedbackjlbetween said z'ampliflers through said controlftubes andtheir'associated circuits, each :of said control tubes an'dffthe -tubes ofsaiti ami plifiers having small thermal capacity filaments.

' amplifiers, .sa'id sourceso'f current"'bingjlisolated said control tubes and the "tubes of each lo'f said ,fcircuitffor-each amplifier-arran-ged to berespom individualsources of direct current'for eachpol'. 

